The Holy Apostolic and Catholic Assyrian Church of the East is a church that traces its origins to the See of Babylon, said to be founded by the apostle Thomas. It is also called the Assyrian Orthodox Church, but should not be confused with the distinct Syriac Orthodox Church, which belongs to Oriental Orthodoxy. Its geographical origins lie in Iraq and Iran and it onced stretched to Xian in China and Kerala in India. It officially divided from the other Christian churches in 431 AD following the Council of Ephesus which repudiated Nestorianism and as such it is often known as the Nestorian Church, however its theology is not strictly Nestorian. Today there are about 0.5 million members of the church, of which most live in Iraq, Iran and Syria.

The Assyrian Church is the original Christian church in what was once Parthia; today Iraq and western Iran. Geographically it stretched in the medieval period to China and India: a monument found in Xi'an, the Tang-period capital of China, in Chinese and Syriac described the activities of the church in the 7th and 8th centuries, while half a millennium later a Chinese monk went from Beijing to Paris and Rome to call for a crusade with the Mongols against the Mamelukes; prior to the Portuguese arrival in India in 1498, it provided "East Syrian" bishops to the Saint Thomas Christians.

The foundations of Assyrian theology are Diodorus of Tarsus and Theodore of Mopsuestia, who taught at Antioch. The normative Christology of the Assyrian church was written by Babai the Great (551-628) and is clearly different from the dualism that Nestorius was accused of: his main christological work is strikingly called the 'Book of the Union', and in it Babai teaches that the two qnome (essence) are unmingled but everlastingly united in the one parsopa (personality) of Jesus Christ.

The Schism

Although the Assyrian Church split from the western churches in the course of the Nestorian schism, the theology of the Assyrian church can not be defined as Nestorianism. Nestorius, a pupil of Theodore of Mopsuestia and bishop of Constantinople, was condemned because he refused to call the Jesus' mother Mary, the mother of God'. He would only call her 'mother of Christ'. His opponent Cyril of Alexandria accused him of dividing Christ into two persons, which he clearly denied. The affair was complicated by the unclear arguments of Cyril, which soon after provoked the Monophysite schism.

Cyril of Alexandria worked hard to remove Nestorius and his supporters and followers from power. But in the Syriac-speaking world Theodore of Mopsuestia was held in very high esteem, and the condemnation of his pupil Nestorius was not received well. His followers were given refuge. The Persian kings, who were at constant war with Byzantium, saw the opportunity to assure the loyalty of their Christian subjects and supported the Nestorian schism:

They granted protection to Nestorians (462).

They executed the pro-Byzantine Catholicos Babowai who was then replaced by the Nestorian Bishop of Nisibis. Bar Sauma (484).

They allowed the transfer of the school of Edessa to the Persian city Nisibis when the Byzantine emperor closed it for its Nestorian tendencies (489).

The consolidation of the Church

The Christian communities of Mesopotamia had renounced all subjection to Antioch and the "Western" bishops already at the Council of Seleucia in 410, and the Bishop of Ctesiphon assumed the rank of Catholicos. At the time of the arrival of the Nestorian refugees from Edessa, the prelate was Babaeus or Babowai (sometimes also called 'Babai', not to be confused with 'Babai the Great') (457-484), who appears to have received them with open arms. But Bar Sauma, having become Bishop of Nisibis, the nearest important city to Edessa, broke with the weak Catholicos, whom he had deposed at a the Synod of Beth Lapat in April, 484. In the same year Babowai was accused before the king of conspiring with Constantinople and cruelly put to death.

At the synod of Beth Lapat it was also decided that monks and all church dignitaries should marry. This led to apostasy and a weakening of spiritual life, and already by 544 some of the reforms had been reverted. The counter reforms reached their zenith in 571 when Abraham the Great of Kashkar founded a new monastery on Mt. Izla above Nisibis to revive the strict monastic movement, and Henana of Adiabene became head of the school of Nisibis. Henana then broke with the Antiochene tradition of Theodore and openly followed the teaching of Origen. Attempts by the Bishops to censor and condemn Henana failed because of his protection by the royal court and he remained head of the school, even though almost all the students left.

The wars of 610 - 628 between the Persian and Byzantine empires weakened the political standing of the Assyrian church and several sees and villages were lost to the Monophysites. The Assyrian church was not allowed to choose a new Catholicos, and its theological tradition was undermined by Henana. Babai the Great together with Archdeacon Mar Aba administered the church without the authority vested in the position of the Catholicos. But in his official position as 'visitor of the monasteries of the north' Babai had the authority to investigate the orthodoxy of the monks and monasteries of northern Mesopotamia and to enforce discipline. In particular, he drove out married monks.

Babai the Great and his co-religionists worked hard to defend the legacy of Theodore: rival schools were set up in Nisibis and Balad, and the monastery of Mar Abraham, headed by Babai, took in a number of students from the school of Nisibis. Babai himself wrote a great number of commentaries and hagiographies to defeat the Monophysites and the Origenist Henana, and developed the only systematic Assyrian Christology. He taught that the two qnome (essence) are unmingled but everlastingly united in the one parsopa (personality) of Christ.

The defenders were successful: at the episcopal gathering of 612 the teachings of Theodore were canonized. Soon Babai's writings and Christology became normative, and the writings of Henana were doomed to oblivion. Assyrian monasticism was purged and gathered momentum. The church proved to be well organized during the Arab conquest that followed the Byzantine-Persian wars, and flourished for many centuries after.

Eastern expansion

The Assyrian Church was the first Christian tradition to reach China (in 635), and about the same time penetrated into Mongolia, and its relics can still be seen in Chinese cities such as Xi'an. An inscribed stone, set up in February, 781 at Chou-Chih, fifty miles south-west of Sai-an Fu, at the time the capital of China, describes the introduction of Christianity into China from Persia in the reign of Tang Taizong. However when Tang Wu Zon decided to suppress all foreign religions; Christianity largely ceased to exist in China. In 2003, it was discovered that a single church body of the Assyrian Church still existed, cut off from any contact with its Patriarch for centuries.

More recent times

In the 15th century, the church decreed that the title of Patriarch could pass only to relatives of then-patriarch Mar Shimun IV. This upset many in the church's hierarchy, and in 1552 a rival Patriarch, Mar Yohanan Soulaqa VIII was elected. This rival Patriarch met with the Pope and entered into communion with the Roman Catholic Church. The Assyrian Church now had two rival leaders, a hereditary patriarch in Alqosh (in modern-day northern Iraq), and a Papal-appointed patriarch in Diyarbakir (in modern-day eastern Turkey). This situation lasted until 1662 when the Patriarch in Diyarbakir, Mar Shimun XIII Denha, broke communion with Rome, and moved his seat to the village of Qochanis in the Turkish mountains. The Vatican responded by appointing a new patriarch to Diyarbakir to govern the Assyrians who stayed loyal to the Holy See. This became known as the Chaldean Catholic Church. In 1804 the hereditary line of Patriarchs in Alqosh died out, and that church's hierarchy decided to accept the authority of the Chaldean patriarchs.

Assyrians faced reprisals under the Hashemite monarchy for co-operating with the British, and most fled to the West. The Patriarch of Babylon is currently based in Chicago, Illinois, and less than 100,000 of the world's 3.3 million Assyrians remain in Iraq.

The Chaldean community was less numerous at the time of the British Mandate of Palestine, and did not play a major role in the British rule of the country. However with the exodus of Assyrians, the Chaldean Catholic Church became the largest non-Muslim group in Iraq, and many later rose to power in the Ba'ath Party government, the most prominent being Deputy Prime Minister Tariq Aziz.

Today

In 1964, the issue of hereditary succession again caused a schism, with the election of Mar Thomas Darmo as a rival to the hereditary Mar Simon XXIII. Mar Simon resigned in 1973, and was assassinated in 1975 during negotiations over his possible reinstatement. Mar Dinkha IV was elected as Simon's successor, and announced the permanent end of the hereditary succession. While this removes the underlying dispute, the rift between the rival Patriarchs still exists, with Mar Addai as the successor to Mar Thomas Darmo.

On November 11, 1994, an historic meeting of Mar Dinkha IV and John Paul II took place in the Vatican and a Common Christological Declaration was signed. One side effect of this meeting was that the Assyrian Church's relationship to the Chaldean Catholic Church was improved.

There are also large numbers of Assyrian congregations in Iran. In addition, a few remain in Iraq, a single parish exists in China, and the Church has its headquarters (along with four other houses of worship) in Chicago, Illinois, United States.